The present invention relates to tools for the repair of heavy equipment or machinery and, in particular, is directed to apparatus for straightening automobile chassis and the like.
In the repair of an automobile body which has been dented in a severe accident, it is often necessary to exert large forces on selected frame members to straighten or realign the framework. In particular it is often necessary to exert a pushing or pulling force between two points, for example, to correct for skewness in the frame. As used herein the term "straighten" refers to the correction of misaligned or bent frame members, regardless of whether the corrected members assume a "straight" configuration.
Numerous types of hydraulic jacks are known which can be inserted between two points to exert a pulling or pushing force. The known hydraulic jacks are subject to various disadvantages. In some devices, for example, that disclosed in U.S. Pat. No. 3,891,187, a double-acting hydraulic cylinder is employed to produce both a pushing and a pulling action. This type of cylinder provides for both powered extension and powered retraction of a cylinder rod. However, the powered extension is generally stronger than the powered retraction, so that devices driven by such a cylinder will not push and pull with equal force. If the pushing action just meets the minimum requirements for the intended use, then the pulling action will not.
In the process of straightening an automobile chassis or frame member, the hydraulic jack device is itself sometimes subject to strong skew reaction forces. In some devices the driving cylinder is susceptible to damage by these forces. In other devices more complicated, and more expensive, constructions are employed to protect the cylinder.
One device which is almost universally found in autobody shops is the basic hydraulic ram used to exert a pushing force. To some extent, then, a self-contained unit providing both a pushing and a pulling force represents a duplication of equipment and an unnecessary expense. Highly desirable and cost-effective is a device for converting pushing action of the basic hydraulic ram into a pulling action. One such device is disclosed in U.S. Pat. No. 3,518,864. That device, however, employs slidably mounted tubular members, which are subject to jamming and damage from skew reaction forces.